The present invention relates to a new circuit design which permits precision triggering of a thyristor in quadrants II and III. In prior art applications, a triac or thyristor is often used to perform phase control of the A/C cycle and in one common application involving outdoor electric lights, it is used to dim the light source. Most prior art applications which use thyristors to dim a light source typically trigger a thyristor in a high side configuration in quadrants I and IV. However, triggering the thyristor in quadrant IV presents many design compromises.
First, in order to trigger quadrant IV, it is well known that additional power is needed to do so. As is stated in the Treccor Electronic Catalog at Pages 133-136, which is herein incorporated by reference, typically twice as much power is needed to trigger quadrant IV as compared with quadrants I-III. Moreover, as the temperature drops, the power required to trigger the thyristor in quadrant IV increases. Often, in outdoor lighting applications, once the outside temperature reaches 0.degree. C. to -20.degree. C., the power supply of the lighting device is often incapable of supplying the power needed to trigger quadrant IV of the triac, which results in the device shutting down.
To overcome this problem, outdoor lighting products may use larger power supplies but this increases the cost of the unit and increases the operating temperature. On the other hand, some manufacturers accept this limitation with the knowledge that at low temperatures their units will cease functioning. Consequently, there is a need for circuitry which will enable a thyristor to be triggered in quadrants II and III with their associated lower power requirements and increased temperature tolerance.